As is known in the art, a radio frequency (RF) amplifier circuit receives an RF signal at an input thereof and provides an amplified version of the RF signal at an output thereof. Ideally, the RF input signal is provided having a single frequency (the so-called fundamental frequency) and does not include any harmonic frequently signal components. Similarly, ideally, the RF amplifier provides an amplified version of the fundamental frequency RF signal and does not generate any harmonics as a result of the amplification. In practical RF amplifier circuits however, harmonic frequency signal components exist.
When an RF input signal reaches a particular power level, the RF amplifier begins to operate in a non-linear amplification region. This results in the generation of harmonic frequency signal components. Thus, in some cases, it is desirable to provide the amplifier circuit with an RF signal path, sometimes referred to as bypass path, which can be used when amplification of a signal is not desired.
In a Class F amplifier configuration, when harmonic frequency signal components are terminated appropriately, the voltage waveform resembles a square wave and the overlap with the current waveform (conduction angle) becomes minimal. By virtue of the minimal conduction angle (the DC power dissipated), the efficiency can be very high with the theoretical maximum being 100% efficiency.
The Class F amplifiers are conventionally designed with harmonics terminated at the amplifier as follows: (a) a short circuit impedance characteristic is presented to even harmonics at an amplifier output terminal; (b) an open circuit impedance characteristic is presented to odd harmonics at the amplifier output terminal (except the fundamental signal frequency); (c) an output impedance characteristic of the amplifier is matched to a load at the fundamental signal frequency; and (d) a short circuit impedance characteristic is presented to harmonics at an input terminal of the amplifier.
The short circuited harmonics at the input are important because all high efficiency analysis assumes a sinusoidal drive at the amplifier input port. However, the amplifier input impedance can be as non-linear as the output impedance and a signal waveform can be easily distorted at the input, degrading the efficiency. Thus, to maintain a sinusoidal waveform at the input, all harmonics should be shorted.
As is also known, inverse Class F is a dual to standard Class F, where current and voltage waveforms are exchanged. This is realized by selecting the load conditions as follows: (a) open circuited even harmonics at the device output terminal; (b) short circuited odd harmonics at the device output terminal except for the fundamental; (c) matched to the load at the fundamental; and (d) open circuited at all harmonics at device input terminal.
An important harmonic signal in Class F and Inverse Class F is a signal having a frequency equal to a second harmonic of the fundamental signal frequency. The open condition required for the second harmonic for the Inverse Class F is similar to Class E. Therefore, there is a lot of commonality between Class E and Inverse Class F amplifiers.
As is known in the art, it is sometimes desirable to configure an RF amplifier circuit in a push-pull configuration. Balun circuits (or more simply “baluns”) are often used with push-pull amplifiers to link a symmetrical (balanced) circuit to an asymmetrical (unbalanced) circuit.